1. Field of the Invention
The present invention relates to melt retention and crystal growth with respect to an oxide, and more particularly, it relates to methods of melt retention and crystal preparation in relation to a yttrium-based or lanthanoid element-based oxide superconductor.
2. Description of the Background Art
A Y-based oxide superconductor (YBa.sub.2 Cu.sub.3 O.sub.7-x, hereinafter referred to as Y123) is watched with interest as a high temperature superconducting material having a critical temperature of 90K. Establishment of a technique for preparing a large-sized single crystal substrate is desired for preparation of a superconducting electron device thereof. However, it is considerably difficult to obtain a large-sized single crystal of Y123, since Y123 results from peritectic solidification reaction and it is difficult to retain a BaO--CuO-based melt serving as a flux in a crucible due to high reactivity.
In general, a flux method is suitable for preparing a crystal by peritectic solidification reaction. Also in preparation of a Y123 crystal, therefore, crystal growth by the flux method has been mainly attempted with employment of a crucible of alumina or platinum and a flux of a BaO--CuO-based melt in general. In the flux method, however, it is considerably difficult to stably prepare a large crystal since a number of crystal nuclei are formed in the melt with no controllability. As hereinabove described, it also comes into question that the reactivity of the BaO--CuO-based melt is so high that it is difficult to retain the same in a crucible (references: J. Crystal Growth, 114, 1991, p. 269 by K. Watanabe and J. Crystal Growth, 121, 1992, p. 531 by S. Elizabeth et al.).
On the other hand, the crystal pulling method which is employed for preparing a large single crystal of a semiconductor such as GaAs is suitable for preparing a large single crystal from a melt with excellent controllability. However, it is difficult to apply the pulling method to preparation of Y123 due to the aforementioned problems of peritectic solidification and the reactivity of the melt.
When a Y123 single crystal is grown by the pulling method, retention of the BaO--CuO melt in the crucible comes into question in the first place. One of the largest problems to be solved is related to the type of the material for forming the crucible for storing the melt. A crucible of a noble metal such as platinum is unavailable since its reactivity with the melt is so high that the crucible may be pierced. When the crucible is made of general fire-resistant ceramics, on the other hand, the melt exudes from the crucible or creeps up along the crucible to overflow the same, and hence its volume is extremely reduced.
On the other hand, a crucible which is prepared from an MgO (magnesia) single crystal or a sufficiently dense MgO sintered body having high bulk density can retain a BaO--CuO-based melt. In such a crucible consisting essentially of MgO, it is possible to stably retain the melt with no piercing of the crucible and neither exudation nor overflow of the melt. In this case, however, Mg gradually contaminates the melt with time, to disadvantageously deteriorate characteristics of the crystal as prepared.
Yamada et al. succeeded in continuously growing a Y123 single crystal through the pulling method by employing a yttria crucible and making a Y.sub.2 BaCuO.sub.5 phase coexistent with the melt as a solute supply source (reference: the 54th Scientific Lecture Meeting of the Japan Society of Applied Physics, 1993, 29P-ZK-7, Yamada et al., and Physica C 217 (1993) 182-188, Y. Yamada and Y. Shiohara). It is possible to avoid impurity contamination by employing a crucible consisting essentially of a Y.sub.2 O.sub.3 (yttria) sintered body. However, the yttria crucible has such a disadvantage that the melt overflows the same. FIG. 6 typically illustrates an overflowing state of a melt 63 in employment of a Y.sub.2 O.sub.3 crucible 61. In the state shown in FIG. 6, it is impossible to continue the crystal growth stably over a long period. Applied Physics, Vol. 62, No. 5 (1993) pp. 459 to 462 by Yamada and Shiohara describes that a BaO--CuO melt creeps up along walls of a yttria crucible and overflows the same due to extremely high wettability and reactivity, leading to lowering of the liquid level. This literature also describes that a method of retaining the melt is an important subject for maintaining a constant growing state and implementing long-time growth.
There has hitherto been known no crucible which can simultaneously satisfy stable retaining of a mold and prevention of impurity contamination. Thus, it has been difficult to prepare a large crystal of high quality.